This invention relates to an analyzing apparatus provided with a spectroscopic photometer, and in particular to an analyzing apparatus for measuring transmission light passing through a flow cell, through which a sample to be studied flows, by means of a detecting element for monitoring and a detecting element for sample measurement.
There are known various sorts of analyzing apparatuses, which has a flow cell and in which a spectroscopic photometer is incorporated as a detector. A liquid chromatographic analyzing apparatus, a flowing injection analyzing apparatus and an automatic biochemical analyzing apparatus for clinical use may be enumerated as representatives of this sort of apparatuses.
In an analyzing apparatus having a flow cell a beam splitter is disposed generally between a light source and the flow cell, and influences of variations in the light source are corrected by monitoring reference light taken out therefrom. When such an analyzing apparatus is used e.g. in a liquid chromatograph, variations in base line cannot be eliminated satisfactorily and in particular measurement accuracy is bad in a region, where absorbance is low. It is thought that this may be attributed to the following two reasons.
The first of them concerns variations in the light source. One of the two light beams formed by dividing incident light by means of the beam splitter enters a sample cell and the other the detector on the monitor side. When the light emitting point of the light source fluctuates geometrically, influences of the fluctuations on these two light beams are not identical. Particularly, in the case where the sample cell is a flow cell having a small volume for liquid chromatograph, in general it gives rise to an unbalance that slight geometrical fluctuations in the light emitting point of the light source have large influences on the small volume flow cell side and little influences on the detector on the monitor side having a light receiving surface, which is considerably large with respect to that of the small volume flow cell. Further, if two exit slits of the spectroscopic analyzer are disposed separately for the monitor side and for the sample side or if gaps are disposed intentionally on both the light paths or on one of them, it is extremely difficult to harmonize relation between slight deviations of the optical axis due to the fluctuations in the light emitting point of the light source and the gaps for the two optical paths. Therefore, it is not possible for the prior art techniques to correct satisfactorily errors due to fluctuations in the light source.
The second reason concerns flow of the solution in the flow cell. For example, fluctuations in light transmitted by the flow cell due to variations in the reflective index of the solution flowing through the flow cell often give rise to problems in a photometer for liquid chromatograph. In such a photometer, where light for the monitor is split before the flow cell, it is not possible to eliminate such variations of the transmitted light due to the flow cell. U.S. Pat. No. 4,557,601 has disclosed an analyzing apparatus, in which the transmitted light is measured by means of a spectroscopic photometer after having made light emitted by a light source pass through the flow cell. According thereto, the absorbance is measured by comparing a photometrically measured value at a wavelength corresponding to a peak portion in the absorption spectrum with a photometrically measured value at a wavelength, where the absorbance is small.
The absorbance is represented by logarithm of the ratio S/R of the detected signal S obtained by the detecting element for sample measurement to the detected signal R obtained by the detecting element for monitoring. In this case, it is desirable to use for the signal detected for the monitoring light at a wavelength, where no absorption is produced by the sample flowing through the flow cell. Usually a deuterium lamp is used as the light source in an analyzing apparatus provided with a spectroscopic photometer. In the case where a deuterium lamp is used, the emitted light intensity in its emission spectrum decreases with increasing wavelength, as indicated in FIG. 2. Furthermore, almost all the absorption peaks of samples obtained from liquid chromatograph, etc. are in a wavelength region under 600 nm. Consequently it is desirable to use light having a wavelength longer than 600 nm for the monitor. However, since the detected signal at a certain wavelength is small in the long wavelength region, the value of the denominator is the ratio S/R used for the absorbance calculation, i.e. the detected signal for the monitor is extremely small. The result obtained by this ratio calculation is apt to be influenced by noise and the noise level of data is, therefore, very high.